There are 5 weeks with payments of 10,000; 30,000; 80,000; 200,000; and 500,000 respectively, bringing the total payment to 820,000. For questions like these you should look to the Recettear wiki as it pretty much has all the answers: http://recettear.wikia.com/wiki/Recettear_Wiki

After completing the 5 weeks of payment, there is also an Endless mode (to do whatever you want without having to make payments), New Game+ (a minimum of 7 are required to obtain every item), and a Survival Mode with two difficulty levels (in which you are forced to make payments every week until you [inevitably] are unable to meet the rapidly increasing payments).

The length of a day can vary greatly depending on what you're actually doing (A day spent adventuring will obviously take you much longer than a day of shopkeeping / shopping.)

As for how long the game can potentially last, I'm currently at 130+ hours with 90% completion.I haven't even started playing Survival mode yet.

It took me a slow relaxing 6hours to finish the weeks to payoff the house loans and at the moment just enjoying endless mode. Spoiler Alert! Pleay the other modes to see a few more new characters! *Cuteness in a Game Is a Never Ending Game For Me* So all the best and have fun always! X3